By: Roads & Bridges
The leaders of Omaha, Neb., had a bold plan. They would spend about $22 million building one of the world’s longest pedestrian bridges. It would wind across the Missouri River, connect their community to Council Bluffs, Iowa, link 100 miles of trails for biking and hiking and highlight the skyline.
But in 2004, pride quickly turned to shock. The price tag for the concrete structure came in at $44.9 million, nearly twice the initial estimate.
Rather than rely on the traditional process, where companies submit designs and then contractors bid on the winning proposal, Omaha leaders asked designers and contractors to work together from the start. Submit a design-build plan, they asked, and make sure it doesn’t top our budget.
That’s when designers from HNTB Corp. partnered with contractors from APAC-Kansas Inc., coming up with a new idea for the bridge. It would still be curved and cable-stayed. But their plans would be easier to erect. In addition, their bridge would be easier to repair in the future, if necessary, and it would look more attractive.
The solution? They redesigned the bridge without using concrete. Instead, they chose steel.
Nice curve
Because the steel bridge is so much lighter than concrete, contractors could save money in several ways. They could use smaller cables, for instance. The lighter pylons required a smaller foundation. And construction crews can use the balanced cantilever method, which means building less falsework.
Scott Gammon of APAC, the transportation construction company, said the Missouri River bridge highlights the benefits of designers and contractors working together from the start.
In October 2006, crews in Omaha started building the 2,700-ft-long bridge (2,300 ft are elevated). Two pylons tower more than 200 ft above the river. The bridge meanders in an S-curve from one side of a pylon to the opposite side of the other, but the sections of rolled beams and the precast deck panels are actually straight. That allowed crews to use 23-ft-long beams bought right off the shelf from Nucor-Yamato Steel, without spending time or money heat-curving the metal.
The curved look, meanwhile, comes from making one side of the beams—the outer edge—a little longer.
“Even if you know what you’re looking at, it still looks curved to the eye,” Gammon said. “That was one of the keys to keeping this project so economical. You can attribute that to the flexibility of steel.”
Airing it out
Two other designs for the bridge would have stayed within Omaha’s budget. But the group reviewing the proposals, including city and state leaders and members of the arts community, favored steel.
“The concrete bridge had a heavier appearance to it,” said Foster. “But the steel bridge was a little more wispy, airy—almost looked like, at times, it would be seen as floating above the river.”
The review committee also noted that because the steel bridge’s deck was prefabricated, if repairs were ever necessary decades from now, the work would be much easier to perform.
The bridge opened in September 2008, ahead of schedule and within budget.
